Clutch control for locked power take off shaft during power take off clutch engagement

ABSTRACT

A power take off (PTO) clutch control system for an agricultural tractor is disclosed herein. The PTO clutch is a hydraulic clutch which is operated by a proportional valve capable of pressurizing the clutch with hydraulic fluid at a pressure related to the pulse width of a pulse width modulated (PWM) control signal applied to the valve. The PWM signal is produced by a controller which monitors the input shaft speed and the output shaft speed. A first control signal is applied to the valve when both the input and output shaft speeds are greater than zero. A second control signal is applied to the valve when the input shaft speed has decreased by a predetermined amount and the output shaft speed is substantially zero.

FIELD OF THE INVENTION

The present invention relates to a power take off (PTO) for anagricultural vehicle such as a tractor. In particular, the presentinvention relates to a control system for a locked PTO shaft during PTOclutch engagement.

BACKGROUND OF THE INVENTION

PTOs are used on agricultural vehicles such as tractors to provide powerfor equipment or implements such as combines, mowers and spreaders. Manymodern PTOs are controlled by engaging and disengaging a clutch coupledto the engine of the tractor through an appropriate drive train, tothereby control the power coupled to the PTO shaft. In at least thehigher power tractors, the clutches are hydraulic clutches which areengaged and disengaged using a hydraulic system including appropriatevalves and hydraulic fluid.

A control system for controlling the operation of a PTO clutch toprovide relatively smooth clutch engagement is disclosed in U.S. Pat.No. 5,494,142, entitled "Power Take Off Clutch Engagement ControlSystem". A clutch engagement routine is described in which the pressurein the clutch is allowed to rise gradually which allows the clutch to beengaged slowly and smoothly. However, during operation of a PTO clutch,occasionally the load on the PTO output shaft may be so large that theoutput shaft is prevented from moving, i.e., the speed of the outputshaft is zero. This "locked shaft" condition can be caused, for example,by a plugged implement. If the engine is not large enough to deliver thepower required to unplug the implement, an engine stall can occur.

Accordingly, it would be useful to provide a hydraulic PTO clutchcontrol system which would detect when a locked shaft condition occurs,and instead of following the normal engagement routine in which thepressure in the clutch is gradually allowed to rise, the pressure in theclutch would be increased suddenly and rapidly thereby potentiallyunplugging the locked shaft and allowing resumption of normal operation.In this manner, permanent damage to the shaft can be avoided.

SUMMARY OF THE INVENTION

The present invention relates to a control system for a vehicle such asa farm tractor having a power source for producing rotational motion, aPTO shaft for supplying rotational motion to at least one piece ofequipment other than the vehicle, and a clutch including an input shaftcoupled to the power source and an output shaft coupled to the PTOshaft. The control system includes a clutch control configured to engageand disengage the clutch in response to first and second controlsignals, a first transducer generating an input signal representative ofthe rotational speed of the input shaft, and a second transducer forgenerating an output signal representative of the rotational speed ofthe output shaft. The system also includes a control circuit coupled tothe clutch control, and the first and second transducers. In operation,the control circuit applies first control signals to the clutch controlwhen the rotational speeds of both the input shaft and the output shaftare greater than zero, and applies second control signals to the clutchcontrol when the input shaft speed has decreased by a predeterminedamount and the output shaft speed is substantially zero.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a PTO drive and control system;

FIG. 2 is a schematic block diagram representative of the circuitconfiguration for the controller of the control system;

FIGS. 3A, 3B and 3C are flow charts representative of the controlfunction of the control system;

FIG. 4 is a graphical representation of the status of a control signalapplied to the hydraulic valve of the control system;

FIG. 5 is a graphical representation of the actual and desiredaccelerations of the PTO shaft;

FIG. 6 is a graphical representation of the status of a control signalapplied to the hydraulic valve of the control system during a lockedshaft condition; and

FIG. 7 is a graphical representation of the engine speed and PTO speedduring a locked shaft condition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a power takeoff (PTO) clutch and brake controlsystem 10 for an agricultural vehicle such as a tractor schematicallyrepresented by the dashed line labeled 12 is shown. With the exceptionof the PTO clutch control system 10, tractor 12 may be a conventionalagricultural tractor of the type including an engine 14 havingconventional accessories such as an alternator 16. Engine 14 is thepower source for tractor 12 and, in addition to providing power to thedrive wheels (not shown) of tractor 12, provides the power to applyrotational motion to a multi-plate hydraulically actuated PTO clutch 18.

Control system 10 includes a controller 20 (e.g., a digitalmicroprocessor such as the Intel TN83C51FA), a PTO on/off switch 22, aPTO input clutch speed transducer 24, and an output clutch speedtransducer 26, a PTO status switch 27, a normally closed, solenoidoperated, hydraulic, proportional clutch control valve 28. By way ofexample, transducers 24 and 26 may be variable reluctance sensors;however, signals representative of the rotational speed of the inputshaft of clutch 18 may be derived from alternator 16.

In addition to controlling clutch 18, system 10 may control a hydraulicbrake 30 which inhibits rotational motion of PTO output shaft 32 whenclutch 18 is fully disengaged. System 10 includes a hydraulic valve 34connected to brake 30 by hydraulic conduit 39. Valve 34 engages anddisengages brake 30. Brake 30 is biased to inhibit rotation of shaft 32.Accordingly, valve 34 is normally closed, and opened when brake 30 is tobe released. Depending upon the application and the configuration ofvalve 28 and the hydraulic conduit 36 which connects valve 28 to clutch18, valve 34 may be eliminated by connecting brake 30 directly toconduit 36. Accordingly, as valve 28 applies pressurized hydraulic fluidto engage clutch 18, the pressurized fluid would also release brake 30.By configuring conduits 36 and 39 appropriately, the engagement ofclutch 18 and releasing of brake 30 can be synchronized to avoidengaging clutch 18 without appropriately releasing brake 30.

Transducers 24 and 26 are coupled to digital inputs of controller 20 byelectrical conductors 25 and 29, and conditioning circuits 38 which maybe integral to controller 20. Conditioning circuits 38 filter radio andother undesirable frequencies of interference from the signals producedby transducers 24 and 26 or alternator 16, and introduced in conductors25 and 29. Additionally, circuit 38 places the signals produced bytransducers 24 and 26 or alternator 16 within a 5V range and providesthese signals with a generally squarewave configuration which can beappropriately sampled by controller 20. In operation, transducer 24produces a signal representative of (e.g. proportional to) therotational speed of the input shaft 19 of clutch 18, and transducer 26produces a signal representative of (e.g. proportional to) therotational speed of the clutch output shaft 32. Accordingly, the signalsapplied to controller 20 by transducers 24 and 26 have a generallysquarewave configuration with a frequency proportional to the rotationalspeed of input shaft 19 and output shaft 32, respectively.

Switches 22 and 27 both include an associated conditioning circuit 40and 42, respectively, which may be integral to controller 20. Dependingupon the application, circuits 40 and 42 may provide signal inversionand appropriate filtering to eliminate switch bounce. However, dependingupon the type of controller 20 used, circuits 40 and 42 may beeliminated. The signals produced by switches 22 and 27 are applied todigital inputs of controller 20 via electrical conductors 23 and 31,respectively.

Hydraulic valves 28 and 34 are coupled to digital outputs of controller20 by appropriate amplification and signal conditioning circuits 44 and46 integral to controller 20, and electrical conductors 48 and 50,respectively. As will be discussed in detail below, controller 20applies a pulse-width modulated (PWM) signal to valve 28 via electricalconductor 48 and circuit 44, and applies a digital on/off signal tovalve 34 via electrical conductor 50 and circuit 46. Due to the natureof the solenoids which operate valves 28 and 34, amplification andisolation circuits 44 and 46 are required to produce a control signalhaving sufficient voltage and current to operate valves 28 and 34.Additionally, due to inductive kickbacks which may potentially beproduced by the solenoids of valves 28 and 34, isolation may be requiredin circuits 44 and 46 to protect controller 20.

Turning to the operation of valve 28, valve 28 is a proportionalhydraulic valve which applies hydraulic fluid to clutch 18 from thesystem hydraulic fluid source 52 at a pressure which is related to (e.g.proportional to) the time-averaged voltage applied to the solenoidassociated with valve 28. Thus, the pressure of the fluid applied toclutch 18 via hydraulic conduit 36 by valve 28 may be controlled byapplying a variable voltage signal to valve 28, or may be controlled byapplying a PWM signal to the solenoid of valve 28. Where a PWM signal isapplied to the solenoid of valve 28 to control the pressure of thehydraulic fluid applied to clutch 18, as in the presently preferredembodiment, the pressure of the fluid is proportional to the pulse widthof the PWM signal produced by controller 20.

As discussed above, clutch 18 is a multi-plate hydraulic clutch. Thistype o f clutch is capable of transferring a torque from clutch inputshaft 19 to output shaft 32, where the torque is generally proportionalto the pressure of the hydraulic fluid applied to clutch 18. Shaft 19 iscoupled to engine 14. Shaft 32 is directly coupled to the 1000 RPM PTO(high speed PTO) output shaft 33 (or a high speed output shaft ofanother speed rating such as 750 RPM), and is coupled to the 540 RPM PTO(low speed PTO) shaft 35 by a reduction gear 37. Accordingly, the torquetransferred between shafts 19 and 32 will be generally proportional tothe pulse width (duty cycle) of the PWM signals applied from controller20 to the solenoid of valve 28. Ideally, it may be convenient to havethe torque transferred between shafts 19 and 32 exactly proportional tothe pulse width of the PWM signals applied to the solenoid of valve 28;however, in mechanical systems, such a relationship is difficult toobtain. Accordingly, controller 20 is programmed to compensate for theinability to obtain such proportionality, and overall non-linearity inthe electronics and mechanism of the control system 10.

Referring to FIG. 2, controller 20 includes a memory circuit 54 havingRAM and ROM, and is configured (programmed) to provide the operations ofa speed sensing circuit 56, a timing circuit 58, a switch statusmonitoring circuit 60, a signal processing circuit 62, and a valvecontrol signal output circuit 64. The direction and channels for dataflow between circuits 54, 56, 58, 60, 62 and 64 are shown in FIG. 2. TheROM of memory circuit 54 stores those values required for system 10initialization, and the constants required for the operation of certainprograms run by controller 20. The RAM of memory 54 provides thetemporary digital storage required for controller 20 to execute thesystem program.

Speed sensing circuit 56 receives the signals from transducers 24 and 26which are applied to conductors 25 and 29, and converts the signals todigital values representative of the rotational speeds of shafts 19 and32, respectively. Timing circuit 58 includes counters which are utilizedby signal processing circuit 62 while executing the programmingrepresented by the flow charts of FIGS. 3A, 3B and 3C. Switch statusmonitoring circuit 60 converts the signals applied by switches 22 and 27to conductors 23 and 31 to digital values representative of the statusof these switches.

Valve control signal output circuit 64 produces a 400 Hz PWM signalapplied to the solenoid of valve 28 via conductor 48 and isolationcircuit 44 having an appropriate pulse width, and produces the on/offsignal applied to valve 34 via conductor 50 and circuit 46. As brieflydiscussed below, the program executed by controller 20 is executed at100 Hz. Thus, the pulse width of the signal produced by circuit 64 isupdated every 10 milliseconds or every 4 cycles of the PWM signal.

The operation of signal processing circuit 62 will now be described indetail in reference to FIGS. 3-7. (FIGS. 3A, 3B and 3C represent theoperational steps of the program run by controller 20.) Upon startup(step 66), controller 20 reads the ROM of memory circuit 54 andinitializes the counter in timing circuit 58 to a number of countsrepresentative of 6 seconds which corresponds to the main timing counterreferenced in steps 73, 80, 86 and 84 below. In addition, controller 20initializes those other variables and constants which may be utilized inthe programming of controller 20 (step 68). In step 70, circuit 62 readsthe digital value representative of the status of PTO switch 22 fromcircuit 60, and returns if switch 22 has not been closed. If switch 22is closed, after it was detected open, circuit 60 executes the stepsrequired to begin engagement of clutch 18.

In step 71, circuit 62 accesses circuit 60 to determine if switch 22 wasopened and closed. If switch 22 was opened and closed, circuit 62 setsthe counts in timing circuit 58 to a number representative ofapproximately 2 seconds (step 73). If switch 22 was not opened andclosed, circuit 62 advances to step 72.

In step 72, circuit 62 reads the digital value representative of thestatus of switch 27 from circuit 60 and determines whether or not thePTO is operating as a 1000 RPM PTO or a 540 RPM PTO. If switch 27produces a signal representative of a 540 RPM PTO (low speed), a LOW PTOflag is set. In step 74, circuit 62 determines whether or not the LOWPTO flag is set. If the LOW PTO flag is set, circuit 62 calculates thetorque limit for clutch 18 at step 75 and stores a value in the RAM ofcircuit 54 representative of the maximum pulse width of the PWM signalto be applied to the solenoid of valve 28 during operation of the 540RPM PTO. The maximum pulse width depends upon the configuration oftractor 12, and is set so that the torque transferred by clutch 18 isless than the maximum torque at which the 540 RPM PTO shaft will fail.

Since the reduction required to reduce the speed of the 540 RPM shaft toapproximately 50% of the 1000 RPM shaft is approximately 2 to 1, atorque is applied to the 540 RPM shaft which is approximately twice aslarge as the torque which can be applied to the 1000 RPM shaft for agiven engine torque. Accordingly, the maximum pressure applied to theclutch through the valve 28 during operation of the 540 RPM shaft totransmit the same torque is approximately 50% of the maximum pressureapplied to the clutch through the valve 28 during the operation of the1000 RPM PTO shaft. This pressure is controlled by changing the width ofthe PWM signal applied. The maximum pulse width value of the PWM signalassociated with the 540 RPM PTO shaft is stored in the ROM of circuit54. At step 74, if circuit 62 determines that the LOW PTO flag is set,circuit 62 will utilize the maximum pulse width value stored in circuit54 which is associated with the maximum torque clutch 18 can transferbetween shafts 19 and 35 during operation of the 540 RPM PTO shaft,without causing failure of the 540 RPM PTO shaft due to torque overload.

In step 76, circuit 62 reads the digital values representative of therotational speeds of input shaft 19 and output shaft 32 from circuit 56.In step 77, a determination is made whether or not the input shaft speedis greater than zero. If not, this indicates that engine 14 is notrunning, and a return is executed. If the input shaft speed is greaterthan zero, processing proceeds to step 78. In step 78, circuit 62compares the speeds of shafts 19 and 32. If the shaft speeds are thesame, circuit 62 resets timing circuit 58 to a count representative of 2seconds, and sets a STEADY STATE flag (step 80). Subsequently, circuit62 loops to execute step 102 and the steps beginning at step 100. Atstep 102 the pulse width value is increased by 1.00%.

In step 82, circuit 62 determines whether or not the STEADY STATE flagis set. If the STEADY STATE flag is set, circuit 62 determines if thespeed difference between shafts 19 and 32 is greater than five percent(5%) (step 83), the timer counter is decremented by 2.5 milliseconds(step 84), and circuit 62 jumps to the programming associated with steps100. If the STEADY STATE flag is not set, circuit 62 goes to step 86. Ifthe speeds of shafts 19 and 32 are different, circuit 62 decrements thecounter of circuit 58 by counts representative of 2.5 milliseconds (step86). (The programming represented by the flow charts of FIGS. 3A, 3B and3C runs at a rate of approximately 100 Hz. Accordingly, to decrement thetimer counter in circuit 58, the counter must be decremented by thenumber of counts associated with 10 milliseconds.)

In step 88, circuit 62 reads the value representative of the rotationalspeed of output shaft 32 to determine whether or not shaft 32 is moving.If shaft 32 is moving, circuit 62 applies a digital signal to circuit64, where circuit 64 responds to the signal by applying a signal toconductor 50 which causes valve 34 to release brake 30 (step 92). Ifshaft 32 is not moving, at step 89, a determination is made whether ornot the input shaft speed has decreased by more than 150 RPM. If so,this indicates engine droop, and processing proceeds to step 91. If theinput shaft speed has not decreased by more than 150 RPM, processingproceeds to step 90. At step 90, circuit 62 reads the time from timercircuit 58 associated with the times since the PTO switch was closed andsets the pulse width value to a predetermined percentage (e.g. 20%) ofthe maximum pulse width value either set at step 75 in the case ofoperation at 540 RPM, or read from circuit 54 in the case of operationat 1000 RPM, if switch 22 has been closed for 300 milliseconds or less.If the time is greater than 300 milliseconds, the pulse width value isincreased by 0.1% for each 10 millisecond increment of time elapsedsubsequent to switch 22 being closed for 300 milliseconds. After settingthe pulse width value at step 90, circuit 62 jumps to step 104.

At step 91, (if engine droop has occurred), the pulse width is set tothe maximum, corresponding to the maximum clutch engagement pressure.Processing then proceeds to step 104.

In general, steps 88 and 90 are provided to produce smooth engagement ofclutch 18. More specifically, before the plates of clutch 18 engage, acertain volume of hydraulic fluid must be provided to clutch 18 beforethe clutch plates of clutch 18 travel through the distance required toengage the clutch plates. During this clutch filling process, it isundesirable to apply hydraulic fluid to the clutch at a fixed orundesirably high pressure since the clutch will abruptly apply torquefrom shaft 19 to shaft 32. Such an abrupt application of torque canpotentially cause damage to shaft 32 or an associated implementconnected to the PTO output shaft. By initiating the filling of clutch18 with a pressure equivalent to the pre-stress force applied by theclutch springs, the clutch plates move relatively slowly towardengagement, and the pressure is increased gradually until engagement.This process prevents the abrupt transfer of torque from shaft 19 toshaft 32.

FIG. 4 illustrates the pulse width of the PWM signal plotted againsttime. As shown, the first motion of the output shaft occurs at time T1(i.e., initial clutch engagement), the pulse width of the PWM signalhaving been initiated at a certain % duty cycle (e.g. 20%) at time T0and increased in gradual steps until output shaft 32 begins moving asdetermined at step 88. At time T2, the clutch is fully locked up.

FIGS. 6 and 7 illustrate the case when the output shaft of tractor 12does not rotate, for example, due to a plugged implement or heavy load.As shown in FIG. 7, the speed of the input shaft is decreased by theload on the output shaft. Controller 20 monitors the speeds of the inputand output shafts (in steps 88 and 89) and when the output shaft is notrotating and the engine speed has decreased by a predetermined amount,for example, 150 RPM, the pulse width of the signal on conductor 48 isincreased to the maximum value. This is shown by the sharp increase inFIG. 6 immediately after T1. In this manner, as is illustrated in FIG.7, either an engine kill will occur, or the output shaft will becomeunplugged, leading to resumption of normal operation.

Referring to FIG. 3B, in step 94, circuit 62 calculates a desiredacceleration by dividing the speed at shaft 19 by 2.5 seconds. Ingeneral, step 94 is the start of the process for controlling clutch 18to accelerate output shaft 32 relative to shaft 19 until the speed ofshaft 32 reaches its steady state speed (no clutch 18 slip) which equalsor is proportional to the speed of shaft 19. The acceleration of shaftin step 94 is calculated based upon 2.5 seconds, which was selectedbased upon experimentation to provide optimum acceleration of shaft 32.However, depending upon the system configuration, this time period maybe varied according to the particular tractor and PTO application. Thecalculated acceleration serves as a reference for accelerating shaft 32relative to shaft 19.

In step 96, circuit 62 calculates the shaft acceleration by reading thecurrent speed of shaft 32 from circuit 56, and the speed of shaft 32monitored during the previous loop through steps 70-108. Steps 70-108are executed every 10 milliseconds; thus, the shaft acceleration is thechange in shaft speed between program loops divided by 10 milliseconds.If the actual acceleration of shaft 32 is less than the desired shaftacceleration, the current pulse width is increased by 0.1% (step 98). Ifthe actual acceleration of shaft 32 is greater than the desiredacceleration, the pulse width value is not changed. In certain systems,it may be desirable to reduce the pulse width value when the actualacceleration of shaft 32 is greater than the desired acceleration.However, this type of control may cause hunting, and thus, anacceleration of shaft 32 which is not smooth. Accordingly, in thepresently preferred embodiment of system 10, the pulse width value isnot modified when the actual acceleration of shaft 32 exceeds thedesired acceleration.

Referring again to FIG. 4, the increase in the pulse width value whichoccurs during the execution of steps 94, 96 and 98 is shown betweentimes T1 and T2. As shown, this pulse width value is increasedincrementally at a rate of 0.1% change at each 10 millisecond interval,if the actual acceleration is less than the desired acceleration.Referring to FIG. 5, examples of the desired and actual speeds for shaft32, and engine speed are plotted against time. As shown, at time T1,shaft 32 begins to rotate, and at time T2, the speed of shaft 32 equalsthe speed of shaft 19 (lock-up). At time T2, the speeds of shafts 19 and32 are equal or proportional, and circuit 62 executes steps 100, 101 and102 to ramp up the pulse width value to produce a clutch pressure inclutch 18 associated with the maximum torque to be transmitted betweenshafts 32 and 19. In step 100, the current pulse width value is comparedwith the maximum pulse width value set determined at step 75 in case ofoperation at 540 RPM, and the PWM value stored in circuit 54 in case ofoperation at 1000 RPM. If the current pulse width value set at step 98or step 102 is greater than the maximum pulse width value, the pulsewidth value is set to the maximum pulse width value (step 101).

As discussed above in reference to step 91, just prior to step 104,hydraulic fluid under maximum clutch engagement pressure is applied toclutch 18. In step 104, circuit 62 samples the count of the timer incircuit 58 to determine whether or not the timer has timed out. If thetimer equals 0, then either motion of shaft 32 did not occur within 6seconds (timer count at initialization), or the speed difference betweenshafts 19 and 32 subsequent to time T2 (lock-up) has been greater than5% for more than 2 seconds which indicates undesirable slippage inclutch 18. In step 104, circuit 62 also determines if the speed of shaft19 has gone below 650 RPM. If either the timer count has reached 0 orthe speed of shaft 19 has gone below 650 RPM, circuit 62 sets the pulsewidth to zero (step 105). In some circumstances, the application ofmaximum engagement pressure to clutch 18 for a limited (e.g. 6 second)period of time can clear a plugged or jammed implement coupled to thePTO output.

In step 106, circuit 62 applies the pulse width value generated duringthe execution of steps 70-105 to circuit 64. In response, circuit 64applies a pulse width modulated signal to valve 28 via conductor 48 at afrequency of 400 Hz with a pulse width corresponding to the currentpulse width value which will be updated upon the next execution of steps70 through 106. In step 108, circuit 62 returns to the execution of step70.

Although various features of the control system are described andillustrated in the drawings, the present invention is not necessarilylimited to these features and may encompass other features disclosedboth individually and in various combinations. For example, developmentsin PTO clutches may make electric clutches cost effective for PTOapplications. Accordingly, hydraulic clutch 18 and control valve 28 maypotentially be replaced with an associated electric clutch and electricclutch control circuit.

What is claimed is:
 1. In a vehicle having a power source for producingrotational motion, a power take-off (PTO) shaft for supplying rotationalmotion to at least one piece of equipment other than the vehicle, and aclutch including an input shaft coupled to the power source and anoutput shaft coupled to the PTO shaft, wherein the clutch transmits amaximum torque between the input and output shafts in response to amaximum clutch engagement pressure and transmits a selectable torquebetween the input and output shafts in response to a selected clutchengagement pressure less than the maximum clutch engagement pressure, acontrol system comprising:a first transducer disposed to generate aninput signal representative of the rotational speed of the input shaft;a second transducer disposed to generate an output signal representativeof the rotational speed of the output shaft; a clutch control configuredto engage and disengage the clutch in response to first and secondcontrol signals; and a control circuit coupled to the clutch control,the first transducer and the second transducer, and being configuredto:apply first control signals to the clutch control when both the inputand output shaft speeds are greater than zero, the first control signalscorresponding to a selectable clutch engagement pressure less than themaximum clutch engagement pressure, and apply second control signals tothe clutch control when the input shaft speed has decreased by apredetermined amount and the output shaft speed is substantially equalto zero, the second control signals corresponding to the maximum clutchengagement pressure.
 2. The control system of claim 1, furthercomprising a source of pressurized hydraulic fluid, the clutch being ahydraulic clutch engageable at an engagement pressure related to thehydraulic pressure applied to the clutch, the clutch control including ahydraulic valve for coupling the clutch to the source of pressurizedhydraulic fluid, and the hydraulic valve being a proportional valveconfigured to control the pressure of the fluid applied to the clutchfrom the source.
 3. The control system of claim 2, wherein the controlcircuit includes a digital processor configured to produce the first andsecond control signals which are pulse-width modulated signals having apredetermined frequency, and the pressure applied to the clutch issubstantially proportional to the pulse-width of the modulated signals.4. The control system of claim 2, further comprising:a hydraulicallyoperated brake coupled to the source of pressurized hydraulic fluid anddisposed to inhibit rotation of the output shaft when the hydraulicclutch is disengaged.
 5. The control system of claim 4, wherein thehydraulic valve couples the brake to the source of pressurized hydraulicfluid.
 6. The control system of claim 4, further comprising a hydraulicbrake valve coupled between the source of pressurized hydraulic fluidand the brake.
 7. The control system of claim 1, wherein the first andsecond transducers are magnetic pickups located proximate the input andoutput shafts, respectively.
 8. A tractor comprising:a power sourceconfigured to produce rotational motion; a power take-off (PTO) shaftfor supplying rotational motion to at least one piece of equipment otherthan the tractor; a clutch including an input shaft coupled to the powersource and an output shaft coupled to the PTO shaft, wherein the clutchtransmits a maximum torque between the input and output shafts inresponse to a maximum clutch engagement pressure and transmits aselectable torque between the input and output shafts in response to aselected clutch engagement pressure less than the maximum clutchengagement pressure; a clutch control configured to engage and disengagethe clutch in response to a control signal; a first transducer disposedto generate an input signal representative of the rotational speed ofthe input shaft; a second transducer disposed to generate an outputsignal representative of the rotational speed of the output shaft; and acontrol circuit coupled to the clutch control, the first transducer andthe second transducer, and being configured to apply to the clutchcontrol said control signal representative of the maximum engagementpressure when the input shaft speed has decreased by a predeterminedamount and the output shaft speed is substantially zero.
 9. The tractorof claim 8, further comprising a source of pressurized hydraulic fluid,the clutch being a hydraulic clutch engageable at an engagement pressurerelated to the hydraulic pressure applied to the clutch, the clutchcontrol including a hydraulic valve for coupling the clutch to thesource of pressurized hydraulic fluid, and the hydraulic valve being aproportional valve configured to control the pressure of the fluidapplied to the clutch from the source, wherein the pressure is dependentupon said control signal.
 10. The tractor of claim 9, furthercomprising:a hydraulically operated brake coupled to the source ofpressurized hydraulic fluid and disposed to inhibit rotation of theoutput shaft when the hydraulic clutch is disengaged.
 11. The tractor ofclaim 10, wherein the hydraulic valve couples the brake to the source ofpressurized hydraulic fluid.
 12. The tractor of claim 10, furthercomprising a hydraulic brake valve coupled between the source ofpressurized hydraulic fluid and the brake.
 13. The tractor of claim 8,wherein the control circuit includes a digital processor configured toproduce said control signal which is a pulse-width modulated signalhaving a predetermined frequency, and the pressure applied to the clutchis substantially proportional to the pulse-width of the modulatedsignal.
 14. The tractor of claim 8, wherein the first and secondtransducers are magnetic pickups located proximate the input and outputshafts, respectively.
 15. A tractor comprising:power means for producingrotational motion; a power take-off (PTO) shaft for supplying rotationalmotion to at least one piece of equipment other than the tractor; aclutch including an input shaft coupled to the power means and an outputshaft coupled to the PTO shaft, wherein the clutch transmits a maximumtorque between the input and output shafts in response to a maximumclutch engagement pressure and transmits a selectable torque between theinput and output shafts in response to a selected clutch engagementpressure less than the maximum clutch engagement pressure; clutchcontrol means for engaging and disengaging the clutch in response to acontrol signal; first means for generating an input signalrepresentative of the rotational speed of the input shaft; second meansfor generating an output signal representative of the rotational speedof the output shaft; and controller means for monitoring the first andsecond means for generating, applying the control signal to the clutchcontrol means, the control signal representative of the maximumengagement pressure when the input shaft speed has decreased by apredetermined amount and the output shaft speed is substantially zero.16. The tractor of claim 15, further comprising a source of pressurizedhydraulic fluid, the clutch being a hydraulic clutch engageable at anengagement pressure related to the hydraulic pressure applied to theclutch, the clutch control means including a hydraulic valve means forcoupling the clutch to the source of pressurized fluid, and thehydraulic valve means being a proportional valve configured to controlthe pressure of the fluid applied to the clutch from the source, whereinthe pressure is dependent upon said control signal.
 17. The tractor ofclaim 15, wherein the controller means includes a processor meansconfigured to produce said control signal which is a pulse-widthmodulated signal having a predetermined frequency, and the pressureapplied to the clutch is substantially proportional to the pulse-widthof the modulated signal.
 18. The tractor of claim 15 wherein the firstand second means for generating are magnetic pickups located proximateto the input and output shafts, respectively.
 19. In a tractor includinga power take-off (PTO) shaft for supplying rotational motion to at leastone piece of equipment other than the tractor, and a clutch including aninput shaft coupled to a power source and an output shaft coupled to thePTO shaft, wherein the clutch transmits a maximum torque between theinput and output shafts in response to a maximum clutch engagementpressure and transmits a selectable torque between the input and outputshafts in response to a selected clutch engagement pressure less thanthe maximum clutch engagement pressure, a method for controlling theclutch engagement pressure when the output shaft is prevented frommoving, the method comprising the steps of:a) generating a first signalrepresentative of the rotational speed of the input shaft, b) generatinga second signal representative of the rotational speed of the outputshaft, c) generating a control signal in response to the first signaland the second signal, wherein the control signal is representative ofthe maximum clutch engagement pressure when the input shaft speed hasdecreased by a predetermined amount and the output shaft speed issubstantially zero, d) selectively engaging and disengaging the clutchin response to the control signal.
 20. The method of claim 19 whereinthe control signal is generated as a pulse width modulated signal havinga predetermined frequency, and the pressure applied to the clutch issubstantially proportional to the pulse-width of the modulated signal.